<div class="htmlview paragraph">THE USE OF Rh/Pt catalysts in automobile exhaust emission control is now well established. These catalysts are applicable in both the conventional CO/HC control system and the more novel TWC approach seeking to control CO, HC and NOx emissions simultaneously. During comparative investigations of Rh/Pt vs. Pt catalysts it has become apparent that Rh/Pt catalysts possess certain features which are absent in other noble metal combinations. Thus in general Rh/Pt catalysts show much greater ability to convert NOx under TWC conditions, and produce lower sulphate emissions under net oxidising conditions. On the other hand Rh/Pt can show lower HC conversion and durability than other noble metal combinations when operated under vast excess of oxygen.</div> <div class="htmlview paragraph">Surface studies in this laboratory have revealed that these effects can be attributed in part to surface enrichment of the Rh/Pt system by the rhodium component. This is followed by oxidation to produce durable surface species which are inactive to sulphate formation, and reduce the activity of the catalyst to HC conversion. The role of such species in the conversion of NOx is discussed, and possible solutions to this problem outlined in terms of catalyst design criteria.</div>